Deposition of copper-tin and copper-tin-zinc alloys from an electrolyte

ABSTRACT

The present invention relates to a cyanide-free electrolyte which contains a phosphate and aliphatic or aromatic thio compounds and also to a process for the electrolytic deposition of an alloy of the elements copper and tin and optionally zinc. The electrolyte and the process are characterized in that stannate ions and copper ions and optionally zinc(II) ions and also aliphatic and/or aromatic thio compounds are present in the electrolyte used. The electrolyte can optionally additionally contain carboxylic acids, wetting agents and/or brighteners. The present invention further provides a process for the electrolytic deposition of alloys of copper, tin and optionally zinc on consumer goods and decorative goods using the electrolyte of the invention.

The present invention relates to a cyanide-free electrolyte whichcontains a phosphate and aliphatic or aromatic thio compounds and alsoto a process for the electrolytic deposition of an alloy of the elementscopper and tin and optionally zinc. The electrolyte and the process arecharacterized in that stannate ions and copper ions and optionallyzinc(II) ions and also aliphatic or aromatic thio compounds are presentin the electrolyte used.

The electrolytic deposition of brass (Cu—Zn alloy) and bronzes (Cu—Snalloy) on consumer goods or decorative goods is adequately known. Thesealloys serve, inter alia, as substitute for nickel-containing finishinglayers and are applied, for example, to appropriate substrates inelectrochemical drum coating or rack coating processes.

In the production of brass and bronze layers for the electronicsindustry, the solderability of the resulting layer and possibly itsmechanical adhesive strength are the critical properties. The appearanceof the layers is generally less important than their functionality foruse in this field. In contrast, in the production of bronze or brasslayers on consumer goods, the decorative effect and also durability ofthe layer with an appearance which is as unchanged as possible are theimportant target parameters.

For the production of brass and bronze layers, not only the conventionalprocess using cyanide-containing and thus highly toxic, alkaline bathsbut also various electrochemical processes which can usually be assignedto one of two main groups known in the prior art as a function of thecomposition of their electrolytes are known: processes usingorganosulfonic acid-based electrolytes and processes using diphosphoricacid-based baths. Diphosphoric acid is also referred to aspyrophosphoric acid. Both processes have specific disadvantages whichsignificantly restrict their practical usability. Thus, tin in divalentform is added in both electrolyte systems and oxidizes to ineffectivetin(IV) during operation of the bath, which considerably limits the lifeof the electrolytes. A further restriction arises in the case of thegroup of organosulfonic acid-based electrolytes. These operate in thestrongly acidic pH range and are thus not suitable for particular fieldsof use, for example coating of pressure-cast zinc.

EP 2 032 743 B1 describes an electrolyte for producing Cu—Sn—Zn alloylayers for photovoltaic cells. This electrolyte isphosphate-/pyrophosphate-based and uses tin in tetravalent form asstannate, in contrast to all known cyanide-free systems. Matt Cu—Sn—Znlayers can be deposited from this electrolyte in a very narrow currentdensity window. This type of electrolyte in the form described is notsuitable for production of decorative bronze layers in drum or rackplating.

EP 1 961 840 A1 discloses a nontoxic electrolyte for the deposition ofdecorative bronze alloy layers, which contains the metals to bedeposited in the form of water-soluble salts, with the electrolytecontaining one or more phosphonic acid derivatives as complexing agentsand being free of cyanides, thiourea derivatives and thio derivatives.The electrolyte contains copper and tin or copper, tin and zinc asmetals to be deposited. Tin can be used as divalent or tetravalent tinsalt in this case. Stannates are not disclosed. EP 1 961 840 A1 teachesthat bronze layers which have been deposited electrochemically frombaths with addition of thio compounds have a spotty or matt-veiledappearance and are therefore not suitable for decorative coating ofconsumer goods.

WO 2013/092312 A1 discloses a cyanide-free, pyrophosphate-containingelectrolyte and a process for the electrolytic deposition of a ternaryalloy of copper, tin and zinc. In this case, stannate ions are presentin addition to zinc(II) ions and copper(II) ions in the electrolyte. Itis not possible to produce uniformly white coatings over a wide currentdensity range when using this electrolyte, so that it is unsuitable forcoating decorative articles.

WO 2013/092314 A1 discloses a cyanide-free, pyrophosphate-free andphosphonic acid-free electrolyte and a process for the electrolyticdeposition of a ternary alloy of copper, tin and zinc. In this case,stannate ions are present in addition to zinc(II) ions and copper(II)ions in the electrolyte. As in the case of the electrolyte disclosed inWO 2013/092312 A1, it is also not possible to produce uniformly whitecoatings over a wide current density range when using this electrolyte,so that it is unsuitable for coating decorative articles.

EP 2 071 057 A2 describes a composition for the electrolytic depositionof white bronzes, which contains tin, copper and zinc ions and also atleast one mercaptan selected from the group consisting ofmercaptotriazoles and mercaptotetrazoles. Copper can be present in theform of Cu(I) and Cu(II) salts in the composition according to thatinvention. The tin compounds disclosed are Sn(II) salts. The compositiondoes not contain any phosphates, pyrophosphates or phosphonates. In allexamples, bronzes are deposited at pH values of ≦3.

EP 1 001 054 A2 discloses electrochemical baths for the deposition oftin-copper alloys. The baths comprise a water-soluble tin(II) or tin(IV)salt, a water-soluble copper(I) or copper(II) salt, an organic orinorganic acid or a water-soluble salt thereof and also at least onecompound selected from the group consisting of thioamide and thiocompounds. When sodium stannate(IV) is used, Cu(I) cyanide is also usedat the same time—the electrolytes are thus not cyanide-free. The thiocompounds serve as bath stabilizers or complexing agents. The inorganicacid or the salt thereof can be phosphoric acid, condensed phosphoricacid, viz. pyrophosphoric acid, and phosphonic acid. The electrochemicalbaths according to EP 1 001 054 A2 do not contain any zinc compounds.The electrochemical baths according to EP 1 001 054 A2 allow thedeposition of tin-copper alloys whose appearance can vary as a functionof the copper content, the presence or absence of brighteners and theselected water-soluble metal salts from white to grayish white and frombright to matt.

WO 2010/003621 A1 discloses electrolyte baths for the deposition ofdecorative bronzes, which baths contain copper, tin and optionally zincand also one or more phosphonic acid derivatives, a disulfide and acarbonate or hydrogencarbonate. The tin is present as tin(II) salt inthis case.

It is an object of the present invention to provide cyanide-freeelectrolytes and corresponding processes for the deposition of whitecopper-tin alloys and white copper-tin-zinc alloys, which are able todeposit coatings of uniform color on decorative articles over a widecurrent density range. Said deposition should be able to be broughtabout very optimally with a preferred stoichiometry. The electrolyteshould have a very simple make-up. Furthermore, the process and theelectrolytes of the invention should be superior to the processes andelectrolytes known from the prior art from ecological and economicpoints of view.

These objects and further objects which will be obvious to a personskilled in the art from the prior art are achieved by electrolyteshaving the features of the present claim 1 and by a correspondingprocess as claimed in claim 10. Preferred embodiments of the respectiveinvention may be found in the dependent claims dependent on theseclaims.

The object of providing electrolytes for the deposition of whitecopper-tin alloys and white copper-tin-zinc alloys is achieved accordingto the invention by an aqueous, cyanide-free electrolyte for theelectrolytic deposition of an alloy of copper, tin and optionally zinc,which comprises

at least one salt from the group consisting of phosphates, phosphonates,polyphosphates, diphosphates and mixtures thereofandat least one compound selected from the group consisting of aliphaticand aromatic thio compounds,wherein the metals copper and optionally zinc to be deposited arepresent in dissolved form and tin is present as dissolved Sn(IV) saltand wherein the pH of the aqueous, cyanide-free electrolyte is greaterthan or equal to 9.

It has been found that advantageous copper-tin and copper-tin-zinc alloycompositions can be obtained from the electrolyte described here when atleast one salt from the group consisting of phosphates, phosphonates,polyphosphates, diphosphates and mixtures thereof is present in theelectrolyte in an excess over the copper and tin ions, when a particularratio of copper to tin ions is set at the same time and when tin ispresent at the same time as dissolved Sn(IV) salt. If the electrolyteadditionally contains zinc in order to be able to deposit a ternaryalloy, both zinc and copper are present in dissolved form. Theelectrolyte additionally contains an aliphatic or aromatic thio compoundwhich complexes dissolved Cu salts. The pH of the aqueous electrolyte ofthe invention is greater than or equal to 9 and thus alkaline. The useof aliphatic and aromatic thio compounds in phosphate- and Sn(IV)-basedCu—Sn alloy electrolytes, which optionally additionally contain Zn,makes it possible to complex copper and at the same time promote thecodeposition of tin and optionally zinc in current density ranges from0.1 to 100 A/dm², advantageously from 0.3 to 1.0 A/dm². The usablecurrent density window is thereby considerably widened compared to knownelectrolytes. When the electrolytes of the invention are used, uniformlywhite coatings of copper-tin and copper-tin-zinc bronzes are depositedover a wide working range.

“Uniformly” here means that the coatings have a homogeneous appearance,i.e. same color and same layer properties in respect of gloss, hardnessand corrosion resistance.

Uniformly white coatings of copper-tin bronzes and copper-tin-zincbronzes can be deposited with the aid of the electrolyte composition ofthe invention. The color white can be defined more precisely by means ofan L*a*b* color measurement.

Electrolytes which use stannate as tin source are known for thedeposition of copper-tin and copper-tin-zinc alloys in the prior art,for example EP 1 001 054 A2 as mentioned at the outset. However,stannates are always used in combination with copper cyanides there. Incyanide complexes, Cu is essentially present as Cu(I) cyanide, i.e. as[Cu(CN)₂]⁻. The deposition of Cu—Sn layers from electrolytes containingstannate and Cu(I) cyanide was carried out in EP 1 001 054 A2 at pHvalues in the range from 12 to 13 and led to bronze layers whosestructure and color was nonuniform. In addition, the bronze layersdisplayed burnt deposits. The alkaline baths comprising stannate andCu(I) cyanide also had a poor bath stability.

The use of Sn(II) salts in combination with Cu(I) cyanide instead ofstannate and Cu(I) cyanide likewise does not lead to uniform Cu—Snlayers since Sn(II) is at least partly oxidized to Sn(IV) in thepresence of Cu(I) cyanide, as a result of which the abovementioneddisadvantages of baths containing stannate and Cu(I) cyanide also occurhere.

Phosphates and pyrophosphates are used in the prior art for stabilizingCu—Sn and Cu—Sn—Zn electrolytes, for example in the documents WO2013/092312 A1 and WO 2013/092314 A1 cited at the outset. However, it isnot possible to produce uniformly white coatings over a wide currentdensity range when using these cyanide-free electrolytes based onpyrophosphates or phosphate and stannate. In the case of theelectrolytes disclosed there, the alloy composition is very dependent onthe current density employed. In the relatively low current densityrange, red coatings having a high proportion of Cu and a low proportionof Zn are obtained, while in the high current density range, theproportion of Zn is significantly higher but the coatings are matt gray.These electrolytes are therefore not suitable for decorative coatings.In high pH ranges, i.e. in particular at pH values greater than or equalto 9, there have hitherto been no known cyanide-free electrolytes inwhich Sn(IV) salts are stable and Cu and Sn can be deposited jointly inthe form of uniformly white coatings.

The aqueous electrolytes of the invention and the process for thedeposition of Cu—Sn and Cu—Sn—Zn alloys are explained below, with theinvention encompassing all the embodiments indicated below, bothindividually and in combination with one another.

Copper can be added to the electrolyte in the form of monovalent ordivalent copper salts or mixtures thereof. Any zinc optionally used ispresent in the form of divalent ions in the electrolyte. Under thereaction conditions according to the invention, copper and optionallyzinc are deposited from their water-soluble compounds. Suitablewater-soluble compounds of copper and zinc are selected from the groupconsisting of pyrophosphates, carbonates, hydrogencarbonates, sulfites,sulfates, phosphates, nitrites, nitrates, halides, hydroxides,oxide-hydroxides, oxides and combinations thereof. Halides can befluorides, chlorides, bromides or iodides. Preference is given to usingcarbonates, hydrogencarbonates, sulfates or pyrophosphates of copper andzinc. Particular preference is given to sulfates of copper and zinc. Forthe present purposes, the term “water-soluble” refers to salts of copperand zinc whose solubility in water is at least 0.1 g/l at 25° C.

In an advantageous embodiment, copper is added to the electrolyte in theform of a Cu(I) salt.

In an advantageous embodiment, copper is added to the electrolyte in theform of a Cu(II) salt.

Tin is added to the electrolyte of the invention as Sn(IV) salt, i.e. intetravalent form. Suitable Sn(IV) salts are SnO₂, Sn(OH)₄, SnCl₄, SnBr₄,SnI₄, Sn(SO₄)₂, Sn(NO₃)₄, SnS₂, Na₂SnO₃, K₂SnO₃, K₂SnO₇C₂. In anadvantageous embodiment, the Sn(IV) salt is a stannate. The stannate isadvantageously sodium stannate Na₂SnO₃ or potassium stannate K₂SnO₃. Ina particularly advantageous embodiment, the Sn(IV) salt is sodiumstannate. In a further particularly advantageous embodiment, the Sn(IV)salt is potassium stannate.

The salts of copper, tin and optionally zinc present in the electrolyteof the invention will hereinafter be summarized under the term“bronze-forming salts”.

The electrolyte of the invention further comprises at least one saltfrom the group consisting of phosphates, phosphonates, polyphosphates,diphosphates and mixtures of these salts.

Suitable phosphates are, for example, disodium hydrogenphosphate anddipotassium hydrogenphosphate. A person skilled in the art will knowthat the tribasic phosphoric acid dissociates over three stages and thatboth dihydrogenphosphates and hydrogenphosphates are ampholytes. Theratio of phosphate (PO₄ ³⁻), hydrogenphosphate (HPO₄ ²⁻) anddihydrogenphosphate (H₂PO⁴⁻) ions in a solution is known to depend onthe pH of the solution. For the purposes of the present invention,phosphate, hydrogenphosphate and dihydrogenphosphate ions will thereforebe referred to summarily as “phosphate ions”. In an analogous way, thetribasic phosphonic acid also dissociates over three stages, and bothdihydrogenphosphonates and hydrogenphosphonates are ampholytes. Thesalts of phosphoric acid are referred to summarily as “phosphonates”. Aperson skilled in the art will know that diphosphoric acid andpolyphosphoric acids are also polybasic and the ratio of thecorresponding anions of these acids which are present depends, as in thecase of phosphoric and phosphonic acids, on the pH of the solution. Forthe purposes of the present invention, it is possible to use theirammonium, lithium, sodium and potassium salts, independently of thenumber of hydrogen atoms of diphosphoric acid or polyphosphoric acidsthat have been replaced by ammonium, lithium, sodium or potassiumcations. The compounds from the group consisting of phosphates,phosphonates, polyphosphates and diphosphates which are used in theelectrolyte of the invention are salts of phosphoric acid, phosphoricacid, polyphosphoric acid and diphosphoric acid. The salts here areadvantageously ammonium, lithium, sodium or potassium salts of theseacids. In the case of polybasic acids in which more than one hydrogenhas been replaced by ammonium, lithium, sodium or potassium cations,these cations can be identical or different.

“Mixtures” of phosphates, phosphonates, polyphosphates and diphosphatescan be mixtures of at least two phosphates, at least two phosphonates,at least two polyphosphates or at least two diphosphates. As analternative, these mixtures can be mixtures of at least two compoundsfrom different groups of salts containing phosphorus and oxygen, i.e.,for example, a phosphate and a phosphonate or two phosphates and onediphosphate Phosphates, phosphonates, polyphosphates and diphosphatesare the four groups of salts containing phosphorus and oxygen which areused for the purposes of the present invention.

As indicated above, the phosphates, phosphonates, polyphosphates anddiphosphates are present in excess over the copper and tin ions in theelectrolyte. Here, “excess” means that the sum of the molar amounts ofthe phosphates, phosphonates, polyphosphates and diphosphates is greaterthan the sum of the molar amounts of the copper and tin ions.

In an advantageous embodiment the total concentration of the at leastone salt from the group consisting of phosphates, phosphonates,polyphosphates, diphosphates and mixtures thereof in the electrolyte isfrom 0.05 mol/l to 5.0 mol/l.

In a particularly advantageous embodiment of the present invention, theat least one salt from the group consisting of phosphates, phosphonates,polyphosphates and diphosphates in the aqueous, cyanide-free electrolyteof the invention is a hydrogenphosphate. Particularly suitablehydrogenphosphates are sodium hydrogenphosphate and dipotassiumhydrogenphosphate. In an advantageous embodiment, the electrolytecontains from 20 to 150 g/l of dipotassium hydrogenphosphate.

In a further advantageous embodiment, the electrolyte contains from 20to 150 g/l of disodium hydrogenphosphate.

Suitable pyrophosphates are, for example, sodium pyrophosphate andpotassium pyrophosphate or mixtures thereof. In an advantageousembodiment, the electrolyte contains from 5 to 40 g/l of potassiumpyrophosphate. In a further advantageous embodiment, the electrolytecontains from 5 to 40 g/l of sodium pyrophosphate.

In a further advantageous embodiment, the electrolyte contains from 20to 150 g/l of hydrogenphosphate, preferably 90 g/l of hydrogenphosphate,in the form of disodium and/or dipotassium hydrogenphosphate, and from 5to 40 g/l of pyrophosphate, in the form of sodium and/or potassiumpyrophosphate.

The electrolyte of the invention additionally contains at least onecompound selected from the group consisting of aliphatic and aromaticthio compounds. In an advantageous embodiment, at least one compoundfrom the group consisting of aliphatic and aromatic thio compounds ispresent in a concentration of from 0.02 to 10 g/l in the electrolyte.

Here, “at least one compound from the group consisting of aliphatic andaromatic thio compounds” means that the electrolyte of the inventioncomprises

-   -   precisely one aliphatic thio compound or    -   precisely one aromatic thio compound or    -   at least two thio compounds which are all aliphatic or    -   at least two thio compounds which are all aromatic or    -   at least one aliphatic thio compound and at least one aromatic        thio compound.

Suitable aliphatic thio compounds are, by way of example but notexhaustively, aliphatic carboxylic and sulfonic acids which contain athio group. Suitable aromatic thio compounds are, by way of example butnot exhaustively, pyridine, pyrimidine, pyrazine and hydantoinderivatives which contain a thio group. In an advantageous embodiment,the thio compound is selected from among 2-mercaptopropionic acid,mercaptosuccinic acid, 2-thiopropanedicarboxylic acid, Na3-mercapto-1-propanesulfonate, 2-mercaptonicotinic acid, 2-thiouracil,4,6-dihydroxy-2-mercaptopyrimidine, 2-mercaptopyrimidine,2-thiocytosine, 6-mercaptopyrimidine-4-carboxylic acid,2-mercaptopyrimidin-4-ol, 2-thiohydantoin, 5-sulfosalicylic acid. It isparticularly advantageous to use mercaptosuccinic acid and4,6-dihydroxy-2-mercaptopyrimidine.

In a further advantageous embodiment, the thio compound is selected fromamong from 1 to 10 ml of 2-mercaptopropionic acid, from 0.5 to 10 g ofthiopropanedicarboxylic acid, from 0.05 to 5 g of Na3-mercaptopropanesulfonate, from 0.05 to 5 g of 2-mercaptonicotinicacid, from 0.02 to 5 g of 2-thiouracil and from 0.5 to 10 g of4,6-dihydroxy-2-mercaptopyrimidine, in each case per liter ofelectrolyte.

The pH of the aqueous electrolyte of the invention is greater than orequal to 9. In a particularly advantageous embodiment, the electrolytehas a pH of greater than or equal to 11.

In a particularly advantageous embodiment, the electrolyte of theinvention additionally contains at least one aliphatic saturated orunsaturated dicarboxylic or tricarboxylic acid, an aromatic carboxylicacid, salts and mixtures thereof. “At least one carboxylic acid, saltsand mixtures thereof” means that carboxylic acids and salts thereofmentioned below can be used either individually or in any combination.The aliphatic saturated dicarboxylic acid is advantageously selectedfrom among oxalic acid, malonic acid, succinic acid, glutaric acid,adipic acid, tartaric acid, malic acid. The aliphatic unsaturateddicarboxylic acid is advantageously selected from among maleic acid andfumaric acid. A suitable tricarboxylic acid is citric acid. Suitablearomatic carboxylic acids are, for example, benzoic acid,benzene-1,3,5-tricarboxylic acid and salicylic acid. The salts of thecarboxylic acids mentioned are advantageously the ammonium, lithium,sodium or potassium salts. In the case of salts of polybasic carboxylicacids, the hydrogen atoms of one or more or all carboxyl groups can bereplaced by ammonium, lithium, sodium or potassium ions.

If in the case of polybasic carboxylic acids at least two carboxylhydrogens have been replaced by ammonium, lithium, sodium or potassiumions, these ions can be identical or different. The total concentrationof the carboxylic acids or salts thereof is advantageously from 5 to 100g/l of electrolyte.

In an advantageous embodiment, the carboxylic acid is selected fromamong oxalic acid, tartaric acid and citric acid or the carboxylic acidsalt is selected from among oxalates, tartrates and citrates.

In a particularly advantageous embodiment, the carboxylic acid or saltthereof is oxalic acid or an oxalate. The use of dipotassium oxalateK₂C₂O₄ is very particularly advantageous.

In a further particularly advantageous embodiment, tartaric acid or asalt thereof, for example potassium sodium tartrate, is used.

In a further particularly advantageous embodiment, citric acid or acitrate, for example potassium citrate, is used.

In a further advantageous embodiment, the electrolyte of the inventioncontains at least one further salt. The anions of these salts areselected from the group consisting of sulfates, fluorides, chlorides,bromides, iodides, carbonates, formates, acetates, propionates,butyrates, valerates, nitrates, nitrites, sulfonates, alkylsulfonates,in particular methanesulfonates, amidosulfonates, sulfamates, anions ofaminocarboxylic acids and N-heterocyclic carboxylic acids. The cationsof these salts are selected from among ammonium, lithium, sodium andpotassium ions. In the case of polybasic acids, one or all hydrogenatoms can have been replaced by the cations mentioned. If more than onehydrogen atom has been replaced by one of the abovementioned cations,these cations can be identical or different. The at least one furthersalt will hereinafter also be referred to as “conducting salt”.

In a further advantageous embodiment, the electrolyte of the inventionadditionally comprises at least one brightener. Additions of brightenersto electrolytes for the deposition of bronzes are known to those skilledin the art and can be employed without going outside the scope ofprotection of the claims. The brightener is advantageously selected fromamong bis(3-sulfopropyl) disulfide disodium salt, 3-sulfopropylO-ethyldithiocarbonate potassium salt, 1-(3-sulfopropyl)pyridiniumbetaine, 1-(2-hydroxy-3-sulfopropyl)pyridinium betaine,3-(2-benzothiazole-2-mercapto)propanesulfonic acid sodium salt,S-isothiouronium 3-propanesulfonate, 3-(sulfopropyl)N,N-dimethyldithiocarbamate sodium salt,1-benzyl-3-sodiocarboxypyridinium chloride,3-formyl-1-(3-sulfopropyl)pyridinium betaine, N-(3-sulfopropyl)saccharinsodium salt, saccharin sodium salt, carboxethylisothiuronium betaine,cocoamidopropyldimethylammonium 2-hydroxypropanesulfo betaine,N-(3-cocoamidopropyl-N,N-dimethyl)-N-(3-sulfopropyl)ammonium betaine,6-carboxy-2,4-dihydroxypyrimidine, 2-butenoic acid.

In a further advantageous embodiment, the electrolyte of the inventionadditionally comprises at least one wetting agent. Wetting agentadditions to the electrolytes for the deposition of bronzes are known tothose skilled in the art and can be employed without going outside thescope of protection of the claims. The wetting agent is advantageouslyselected from among

-   -   a cationic, amine polymer having urea groups,    -   a cationic polymer which is made up of the monomers morpholine,        epichlorohydrin and imidazole and has the general formula        (C₄H₉NO)_(x)*(C₃H₅ClO)_(y)*(C₃H₄N₂)_(z),    -   a cationic polymer which is made up of the monomers        epichlorohydrin and imidazole and has the general formula        (C₃H₅ClO)_(x)*(C₃H₄N₂)_(y),    -   N-alkyl-N-(1-oxoalkyl)amino acids and derivatives and salts        thereof    -   and mixtures of these wetting agents.    -   When salts of N-alkyl-N-(1-oxoalkyl)amino acids are used, they        are advantageously the ammonium, lithium, sodium or potassium        salts.

The use of brighteners and wetting agents enables the gloss of the layerto be set in all gradations between silk-matt and high-gloss.

Particularly advantageous embodiments of the present invention areelectrolytes having the following compositions:

General Composition 1:

-   -   bronze-forming salts,    -   at least one salt from the group consisting of phosphates,        phosphonates, polyphosphates, diphosphates and mixtures of these        salts,    -   at least one compound from the group consisting of aliphatic and        aromatic thio compounds.

General Composition 2:

-   -   bronze-forming salts,    -   at least one salt from the group consisting of phosphates,        phosphonates, polyphosphates, diphosphates and mixtures of these        salts,    -   at least one compound from the group consisting of aliphatic and        aromatic thio compounds,    -   at least one aliphatic saturated or unsaturated dicarboxylic or        tricarboxylic acid, an aromatic carboxylic acid, salts and        mixtures thereof.

General Composition 3:

-   -   bronze-forming salts,    -   at least one salt from the group consisting of phosphates,        phosphonates, polyphosphates, diphosphates and mixtures of these        salts,    -   at least one compound from the group consisting of aliphatic and        aromatic thio compounds,    -   at least one conducting salt.

General Composition 4:

-   -   bronze-forming salts,    -   at least one salt from the group consisting of phosphates,        phosphonates, polyphosphates, diphosphates and mixtures of these        salts,    -   at least one compound from the group consisting of aliphatic and        aromatic thio compounds,    -   at least one aliphatic saturated or unsaturated dicarboxylic or        tricarboxylic acid, an aromatic carboxylic acid, salts and        mixtures thereof,    -   at least one conducting salt.

Furthermore, embodiments of the present invention in which theelectrolytes having the abovementioned general compositions 1 to 4additionally contain at least one brightener, at least one wetting agentor at least one brightener and at least one wetting agent areparticularly advantageous. All particularly advantageous embodiments ofthe electrolyte of the present invention are aqueous, cyanide-free andhave a pH of greater than or equal to 9.

According to the invention, the metals copper and optionally zinc arepresent in ionically dissolved form in the electrolyte and tin ispresent as stannate or another Sn(IV) salt. The ion concentration ofcopper is advantageously from 0.05 to 10 g/l, the ion concentration oftin as stannate is advantageously from 0.5 to 40 g/l and the ionconcentration of zinc is advantageously from 0.1 to 10 g/l. It isparticularly advantageous for the ion concentration of copper to be from0.5 to 2.0 g/l of electrolyte, that of tin as stannate to be from 10 to20 g/l of electrolyte and that of zinc to be from 2.0 to 4.0 g/l. Theindicated advantageous ion concentrations of copper, tin and optionallyzinc apply to all abovementioned advantageous embodiments.

The present invention likewise provides a process for the electrolyticdeposition of Cu—Sn and Cu—Sn—Zn alloy layers, in which the substrate tobe coated is dipped as cathode into an electrolyte according to theinvention and current flow is established between the anode and thecathode. It goes without saying that the embodiments named as preferredfor the electrolyte are likewise preferred for the process.

It is advantageous for the proportion of copper in the ternary alloydeposited to be in the range from 20 to 80% by weight, the proportion oftin to be in the range from 10 to 60% by weight and the proportion ofzinc to be in the range from 1 to 30% by weight. Here, the sum of theproportions of all participating metals in the alloy is in each case100% by weight.

In the case of the binary alloy, the proportion of copper is in therange from 30 to 90% by weight and the proportion of tin is in the rangefrom 10 to 70% by weight. The sum of the proportions of allparticipating metals in the alloy is in each case 100% by weight.

In an advantageous embodiment, the ternary alloy deposited is a whitelayer having a proportion of copper of from 50 to 60% by weight, aproportion of tin of 35-45% by weight and a proportion of zinc of 5-15%by weight, where the sum of the proportions of all participating metalsin the alloy is in each case 100% by weight.

The deposited alloy can in all embodiments described here have athickness of 0.4-5 μm, preferably 0.5-3 μm and very particularlypreferably 1-2 μm.

It may be remarked that the alloy composition can likewise change withthe temperature prevailing in the electrolysis. The electrolysis istherefore carried out in the range from 20 to 90° C., preferably from 30to 60° C. and very preferably at about 45° C.

Likewise, the composition of the binary alloy of copper and tin or theternary alloy of copper, tin and zinc can change with the currentdensity set in the electrolysis. It is advantageous to set a currentdensity in the range from 0.1 to 100 ampere per square decimeter. Thecurrent density is preferably from 0.2 to 5.0 ampere per squaredecimeter, very preferably from 0.3 to 1 ampere per square decimeter.

As anode, it is possible to use all electrodes which a person skilled inthe art would consider for this purpose. Preference is given to usinginsoluble anodes (e.g. platinated titanium anodes or mixed metal oxideanodes). In this context, soluble anodes composed of a material selectedfrom the group consisting of electrolytic copper, phosphorus-containingcopper, tin, tin-copper alloy, zinc-copper alloy and zinc-tin-copperalloy or combinations of these anodes can likewise be used.

The electrolyte of the invention and the process of the invention can beused for the electrolytic deposition of alloys of copper, tin andoptionally zinc on consumer goods and decorative goods.

EXAMPLES Example 1 Electrolyte without Addition of a Thio Compound

Basic composition 90 g/l of dipotassium hydrogenphosphate 15 g/l ofdipotassium oxalate 10 g/l of potassium pyrophosphate 10 g/l of Sn assodium stannate 2.0 g/l of Zn as zinc sulfate 0.5 g/l of Cu as coppersulfate pH pH 11.0 Temperature 45° C. Current density 0.3 A/dm²Appearance: gray, matt, nonuniform

Example 2 Electrolyte with 2-mercaptopropionic acid

Basic composition 50 g/l of dipotassium hydrogenphosphate 50 g/l ofdipotassium oxalate 10 g/l of potassium pyrophosphate 10 g/l of Sn aspotassium stannate 2.0 g/l of Zn as zinc sulfate 0.5 g/l of Cu as copper(I) chloride 2 ml/l of 2-mercaptopropionic acid pH pH 10.5 Temperature50° C. Current density 0.5 A/dm² Appearance: white, matt

Example 3 Electrolyte with Na 3-mercapto-1-propanesulfonate

Basic composition 70 g/l of potassium dihydrogenphosphate 15 g/l ofpotassium citrate 20 g/l of potassium pyrophosphate 10 g/l of Sn assodium stannate 10 ml/l of methanesulfonic acid 2.0 g/l of Zn as zincsulfate 0.5 g/l of Cu as copper (I) iodide 1 g/l of Na3-mercapto-1-propanesulfonate pH pH 10.0 Temperature 45° C. Currentdensity 0.4 A/dm² Appearance: white, shiny

Example 4 Electrolyte with Thiopropanedicarboxylic Acid

Basic composition 80 g/l of dipotassium hydrogenphosphate 25 g/l ofdipotassium oxalate 10 g/l of Sn as potassium stannate 2.0 g/l of Zn aszinc sulfate 0.5 g/l of Cu as copper sulfate 2 g/l ofthiopropanedicarboxylic acid 50 mg/l of 3-formyl-1-(3-sulfopropyl)pyridinium betaine pH pH 10.5 Temperature 40° C. Currentdensity 0.5 A/dm² Appearance: white, shiny

Example 5 6-mercaptopyrimidine-4-carboxylic acid

Basic composition 30 g/l of dipotassium hydrogenphosphate 70 g/l ofdipotassium oxalate 10 g/l of Sn as sodium stannate 2.0 g/l of Zn aszinc sulfate 0.5 g/l of Cu as copper sulfate 1 g/l of6-mercaptopyrimidine-4-carboxylic acid 200 mg/l of1-(3-sulfopropyl)pyridinium betaine pH pH 10.5 Temperature 55° C.Current density 0.4 A/dm² Appearance: white, shiny

Example 6 Thiouracil

Basic composition 80 g/l of dipotassium hydrogenphosphate 20 g/l ofdipotassium oxalate 15 g/l of Sn as potassium stannate 3.0 g/l of Zn aszinc sulfate 1.0 g/l of Cu as copper sulfate 4 g/l of 2-thiouracil pH pH11.0 Temperature 45° C. Current density 0.3 A/dm² Appearance: white,shiny, inhomogeneous

Example 7 4,6-Dihydroxy-2-mercaptopyrimidine

Basic composition 50 g/l of dipotassium hydrogenphosphate 25 g/l ofdipotassium oxalate 10 g/l of Sn as sodium stannate 3.0 g/l of Zn aszinc sulfate 1.0 g/l of Cu as copper sulfate 5 g/l of4,6-dihydroxy-2-mercaptopyrimidine pH pH 11.0 Temperature 45° C. Currentdensity 0.4 A/dm² Appearance: white, semigloss

Example 8 4,6-Dihydroxy-2-mercaptopyrimidine

Binary Cu/Sn alloy

Basic composition 30 g/l of dipotassium hydrogenphosphate 70 g/l ofpotassium sodium tartrate 15 g/l of Sn as sodium stannate 1.0 g/l of Cuas copper sulfate 5 g/l of 4,6-dihydroxy-mercaptopyrimidine pH pH 11.0Temperature 45° C. Current density 0.8 A/dm² Appearance: white, matt

1. An aqueous, cyanide-free electrolyte for the electrolytic depositionof an alloy of copper, tin and optionally zinc, which comprises at leastone salt from the group consisting of phosphates, phosphonates,polyphosphates, diphosphates and mixtures thereof and at least onecompound selected from the group consisting of aliphatic and aromaticthio compounds, wherein the metals copper and optionally zinc to bedeposited are present in dissolved form and tin is present as dissolvedSn(IV) salt and wherein the pH of the aqueous, cyanide-free electrolyteis greater than or equal to
 9. 2. The electrolyte as claimed in claim 1,characterized in that it further comprises at least one aliphaticsaturated or unsaturated dicarboxylic or tricarboxylic acid, an aromaticcarboxylic acid, salts and mixtures thereof.
 3. The electrolyte asclaimed in claim 1, characterized in that it additionally contains atleast one further salt, wherein the anion is selected from the groupconsisting of sulfates, fluorides, chlorides, bromides, iodides,carbonates, acetates, formates, propionates, butyrates, valerates,benzoates, nitrates, nitrites, sulfonates, alkylsulfonates,amidosulfonates, sulfamates, anions of amino carboxylic acids andN-heterocyclic carboxylic acids, wherein the cation is selected fromamong ammonium, lithium, sodium and potassium ions.
 4. The electrolyteas claimed in claim 1, characterized in that it additionally contains atleast one brightener selected from the group consisting ofbis(3-sulfopropyl) disulfide disodium salt, 3-sulfopropylO-ethyldithiocarbonate potassium salt, 1-(3-sulfopropyl)pyridiniumbetaine, 1-(2-hydroxy-3-sulfopropyl)pyridinium betaine,3-(2-benzothiazole-2-mercapto)propanesulfonic acid sodium salt,S-isothiouronium 3-propanesulfonate, 3-(sulfopropyl)N,N-dimethyldithiocarbamate sodium salt,1-benzyl-3-sodiocarboxypyridinium chloride,3-formyl-1-(3-sulfopropyl)pyridinium betaine, N-(3-sulfopropyl)saccharinsodium salt, saccharin sodium salt, carboxethylisothiuronium betaine,cocoamidopropyldimethylammonium 2-hydroxypropanesulfo betaine,N-(3-cocoamidopropyl-N,N-dimethyl)-N-(3-sulfopropyl)ammonium betaine,6-carboxy-2,4-dihydroxypyrimidine, 2-butenoic acid.
 5. The electrolyteas claimed in claim 1, characterized in that it additionally contains atleast one wetting agent selected from among a cationic, amine polymerhaving urea groups, a cationic polymer which is made up of the monomersmorpholine, epichlorohydrin and imidazole and has the general formula(C₄H₉NO)_(x)*(C₃H₅ClO)_(y)*(C₃H₄N₂)_(z), a cationic polymer which ismade up of the monomers epichlorohydrin and imidazole and has thegeneral formula (C₃H₅ClO)_(x)*(C₃H₄N₂)_(y), N-alkyl-N-(1-oxoalkyl)aminoacids and derivatives and salts thereof and mixtures of these wettingagents.
 6. The electrolyte as claimed in claim 1, characterized in thatthe metals copper and optionally zinc to be deposited are present inionically dissolved form and the tin is present as Sn(IV) salt, whereinthe ion concentration of copper is in the range from 0.05 to 10 g/l ofelectrolyte, the ion concentration of tin is in the range from 0.5 to 40g/l of electrolyte and the ion concentration of zinc is in the rangefrom 0.1 to 10 g/l of electrolyte.
 7. The electrolyte as claimed in anyof claim 1, characterized in that the compounds of the metals copper andoptionally zinc to be deposited which are water-soluble under the givenconditions are selected from the group consisting of pyrophosphates,carbonates, hydrogencarbonates, sulfites, sulfates, phosphates,nitrites, nitrates, halides, hydroxides, oxide-hydroxides, oxides andcombinations thereof.
 8. The electrolyte as claimed in claim 1,characterized in that the dissolved Sn(IV) salt is a stannate.
 9. Theelectrolyte as claimed in claim 1, characterized in that the thiocompound is selected from among 2-mercaptopropionic acid,mercaptosuccinic acid, 2-thiopropanedicarboxylic acid, Na3-mercapto-1-propanesulfonate, 2-mercaptonicotinic acid, 2-thiouracil,4,6-dihydroxy-2-mercaptopyrimidine, 2-mercaptopyrimidine,2-thiocytosine, 6-mercaptopyrimidine-4-carboxylic acid,2-mercaptopyrimidin-4-ol, 2-thiohydantoin, 5-sulfosalicylic acid.
 10. Aprocess for the electrolytic deposition of an alloy of the elementscopper and tin and optionally zinc, wherein the substrate to be coatedis dipped as cathode into an electrolyte as claimed in claim 1 andcurrent flow is established between the anode and the cathode.
 11. Theprocess as claimed in claim 10, characterized in that the proportion ofcopper in the alloy is in the range from 20 to 80% by weight, theproportion of tin is in the range from 10 to 60% by weight and theproportion of zinc is in the range from 1 to 30% by weight, where thesum of the proportions of all participating metals in the alloy is ineach case 100% by weight.
 12. The process as claimed in claim 10,characterized in that the proportion of copper in the alloy is from 50to 60% by weight, the proportion of tin is from 35 to 45% by weight andthe proportion of zinc is from 5 to 15% by weight, where the sum of theproportions of all participating metals in the alloy is in each case100% by weight.
 13. The process as claimed in claim 10, characterized inthat the proportion of copper in the alloy is in the range from 30 to90% by weight and the proportion of tin is in the range from 10 to 70%by weight, where the sum of the proportions of all participating metalsin the alloy is in each case 100% by weight.
 14. The process as claimedin claim 10, characterized in that the electrolyte is kept in the rangefrom 20 to 90° C.
 15. The process as claimed in claim 10, characterizedin that a current density in the range from 0.1 to 100 ampere per squaredecimeter is set.
 16. The process as claimed in claim 10, characterizedin that insoluble anodes (e.g. platinated titanium anodes or mixed metaloxide anodes) or soluble anodes composed of a material selected from thegroup consisting of electrolytic copper, phosphorus-containing copper,tin, tin-copper alloy, zinc-copper alloy and zinc-tin-copper alloy orcombinations of these anodes are used.